1. Field of the Invention
The present invention relates to a two-wire transmitter starter circuit and a two-wire transmitter including the two-wire transmitter starter circuit. More specifically, the present invention relates to a two-wire transmitter starter circuit including a simplified circuit configuration and a two-wire transmitter including the two-wire transmitter starter circuit.
Priority is claimed on Japanese Patent Application No. 2012-250104, filed Nov. 14, 2012, the content of which is incorporated herein by reference.
2. Description of Related Art
Generally, a two-wire transmitter includes a starter circuit to secure supply of power and achieve stability of the transmitter at the time of start-up. FIG. 4 is a circuit diagram illustrating an example of a two-wire transmitter starter circuit of the related art.
In FIG. 4, a two-wire transmitter 10 is connected to two-wire type transmission lines L1 and L2. The two-wire transmitter 10 and the transmission lines L1 and L2 are formed as a smart (smart) or fieldbus (foundation, Profibus). The smart (smart) and fieldbus (foundation, Profibus) mean a standardized transmission system to interconnect each control devices and each field devices.
The two-wire transmitter 10 includes a starter circuit 20, a current conversion unit 30, a power supply unit 40, and an internal circuit 50.
The starter circuit 20 includes an emitter of one transistor Q1 configuring a current mirror connected to the transmission line L1 via a resistor R1, and an emitter of the other transistor Q2 connected to the transmission line L1 via a resistor R2. A collector of the transistor Q1 is connected to a power supply line Vcc having a power supply voltage Vcc. A collector of the transistor Q2 is connected to a connection point of a base of the transistor Q1 and a base of the transistor Q2. A connection point of the base of the transistor Q1, the base of the transistor Q2, and the collector of the transistor Q2 is connected to a common potential COM via a serial circuit of a switch SW and a constant current source I1.
The current conversion unit 30 is configured to convert a detection signal of a sensor, which is not illustrated, to a DC current ranging from 4 to 20 mA, and includes a current source I2 having one terminal connected to the transmission line L1 and the other terminal connected to the power supply line Vcc, a resistor R3 connected in series with the transmission line L2, a voltage measurement unit V configured to measure a voltage applied across the resistor R3, and a current source control unit CTL configured to control the current source I2 so that an output current has a predetermined value based on a voltage measurement result of the voltage measurement unit V.
A serial circuit of a resistor Ra and a resistor Rb is connected between the power supply line Vcc and the common potential COM. A non-inversion input terminal of a comparator CMP is connected to a connection point of the resistor Ra and the resistor Rb, and an inversion input terminal of the comparator CMP is connected to a reference voltage source Vref1. An output signal of the comparator CMP turns the switch SW on or off.
The power supply unit 40 is connected in parallel with the serial circuit of the resistor Ra and the resistor Rb. The power supply unit 40 includes, for example, a Zener diode ZD. An anode of this Zener diode ZD is connected to the common potential COM, and a cathode of the Zener diode ZD is connected to the power supply line Vcc. Further, the power supply unit 40 may include an error amplifier or the like in place of the Zener diode.
The internal circuit 50 includes, a sensor which is not illustrated and is configured to measure a physical amount such as a pressure or a temperature, a microprocessor which is not illustrated and is configured to perform predetermined signal processing on an output signal of the sensor, and the like. This internal circuit 50 is connected in parallel with the power supply unit 40 and is configured to be driven by an output voltage of the power supply unit 40.
Operations of the circuit illustrated in FIG. 4 will be described. An internal power supply voltage of the power supply line Vcc is Vcc as described above, a voltage of the transmission line L2 is Va, a voltage of the transmission line L1 is Vb, a voltage of a connection point of the base of the transistor Q1, the base of the transistor Q2, the collector of the transistor Q2 and the output terminal of the comparator CMP is Ve, and a voltage of a connection point of the resistor Ra, the resistor Rb and the non-inversion input terminal of the comparator CMP is Vn.
Before start-up, the switch SW is on, and the constant current source I1 operates to flow a predetermined current even when the internal power supply voltage Vcc has not yet launched, in other words, a predetermined voltage has not yet been set to the internal power supply voltage Vcc. Accordingly, a non-inversion input voltage Vn of the comparator CMP is pulled down by the resistor Rb, and voltage Vn<voltage Vref1, such that an output of the comparator CMP becomes low and the voltage Ve becomes low.
At the time of start-up, the transistors Q1 and Q2 are both turned on, a collector current Ist of the transistor Q1 flows due to a rise of the voltage Vb. The collector current Ist promotes the rise of the internal power supply voltage Vcc. Further, the resistors R1 and R2 suppress an upper limit of the collector current Ist.
Further, when the internal power supply voltage Vcc becomes a predetermined voltage and the voltage Vn becomes a predetermined voltage (voltage Vn>voltage Vref1), the output of the comparator CMP becomes high, the voltage Ve becomes high, and the transistors Q1 and Q2 are both turned off. In other words, when the output voltage of the current conversion unit 30 is greater than or equal to a predetermined value, the starter circuit 20 stops.
The transistors Q1 and Q2 are both turned off during a normal operation. Specifically, since the internal power supply voltage Vcc is sufficiently high, the voltage Vn becomes sufficiently high (voltage Vn>Voltage Vref1), the output of the comparator CMP become high and the voltage Ve becomes sufficiently high. As a result, the base currents of the transistors Q1 and Q2 do not flow and the collector currents of the transistors Q1 and Q2 do not flow.
The configuration of a two-wire transmitter starter circuit which is similar to that in FIG. 4, is described in Japanese Unexamined Patent Application, First Publication No. 2006-174236.
According to the circuit configuration illustrated in FIG. 4, a constant current source I1 operating even when the internal power supply voltage Vcc has not yet launched need to be prepared. Therefore, the circuit configuration illustrated in FIG. 4 is complicated.